The invention relates to a signal interface for a voltage input signal.
In industrial instrumentation, industrial electric drives, such as motor drives and frequency converters, and also in other drives there is often a need for analog or digital signals that signal various quantities relating to the operation of the drive. Such signals may include, for example, control signals and measurement signals. Signal sources may include a rotatory switch or a push button, a guard switch, sensors etc. The voltage level and voltage type depend on the signal source but vary typically from DC voltage of 0 to 10 VDC to alternating voltage (AC) of 230 VAC of different switch signals.
Conventionally, analog and digital signals have been separated by forming them already on the hardware level as separate groups of input channels in such a way that the analog channels are converted into digital form with multiple-input SAR (Successive Approximation Register) type A/D converters, and digital inputs typically utilize optoisolators to provide galvanic isolation from the common potential of the channels. To improve resolution, the measurement range of the analog inputs is typically dimensioned to ten volts, whereby special measures have had to be used for analog voltages higher than this. Correspondingly, digital channels have typically been intended for one single nominal voltage level, i.e. either for a DC level of 24 volts or in special cases for an AC level of 230 volts.
In digitizing, a converter of the SAR type has, however, several disadvantages, a few of which are presented in the following. 1) Even a short interference peak affecting an analog signal to be measured at the sampling moment may lead to a greatly erroneous digitized value because the sampling may last only a few nanoseconds. 2) To prevent interference the measuring circuit must be provided with hardware filters, and on the digital signal processing side, digital filters and discriminators of different levels must still be arranged. 3) Conventionally, the analog signals to be digitized are bound to the same ground potential, whereby loop currents are easily generated and may cause mains-frequency common-mode interference that is difficult to prevent. 4) Fast SAR converters capable of over 12-bit conversion are expensive, so the input channels must be adapted for operation in a given limited voltage range in order for the resolution of the digitization to be sufficient. 5) Hardware filtering provided due to interference usually prevents measurement of digital input signals by means of a high-speed SAR converter.
U.S. Pat. No. 4,316,132, U.S. Pat. No. 5,349,351 and U.S. Pat. No. 7,126,514 disclose multiplexing of analog control signals to one or more A/D converters.
For digital input signals, in turn, the galvanic isolation has conventionally been provided by using optoisolators, which enable safe detection of even 230-volt voltage levels. Optoisolators have, however, several disadvatages, a few of which are presented in the following: 1) the relatively high current level required by LED on the input side (5 . . . 10 mA), which results in either very limited input voltage ranges or in a complex and expensive constant-current generator; 2) this results in loading of a signal source, which is not always acceptable; 3) a great power loss at a high input voltage; and 4) a need for separate rectification when alternating voltages are indicated, and also for a comparator if it is desirable to transfer polarity information.
To keep the costs at a reasonable level, the input channels have typically been positioned, according to standard procedure, as part of a particular motor control board, which is often positioned close to the power stage of the frequency converter or even constructed as part of it. This involves several disadvantages.